1. Field of the Invention
The present invention relates generally to a method and apparatus for correcting skew of a sheet being transported along a path of travel to a processing station. More particularly, the invention relates to transporting a sheet containing a printed image and correcting the alignment of a leading edge of the printed image relative to the entrance of a processing station such as a cutter for cutting the image from the sheet.
2. Description of Related Art
The commercial printing of photographic images on a continuous roll of photographic paper is becoming more and more common. In many such applications, a plurality of images from a customer's order are printed in aligned rows on a segment of the roll and then the segment is cut from the roll. This cutting, sometimes referred to as “post cutting”, can create leading and trailing edges that are not precisely aligned with the printed images. These skewed leading and trailing edges cannot be used to align the printed images for subsequent processing, especially for the machine controlled cutting of individual printed images from the segment. In this respect, relying on a leading edge of the segment to orient the entry of the segment into a cutter will result in the cutter likely severing a portion of the printed image when a cut is made.
Accordingly, while it is possible to rely on a leading edge of the segment to orient the segment relative to a cutter, this can be done only in cases where the printed image itself has an aligned relationship with the leading edge. This cannot be done in cases where the leading edge of the segment is skewed relative to the printed image on the segment. Instead, some other reference is required.
Another problem associated with deskewing is maintaining the position of one portion of the segment while correcting the position of another portion in order to bring the two into alignment. For example, pivoting the segment can result in a translation or movement of the pivot point. Movement of the pivot detracts from properly deskewing the segment. Holding a portion of the segment in a fixed position to prevent the pivot point from wandering can cause a wrinkle, bow, or damage to the segment as one portion of the segment is forced to skew about a fixed portion of the segment.
Deskewing sometimes is accomplished by over driving a lagging side of a segment while slowing the advance of a leading side. A drawback here is that the advance of a sheet must be monitored at a number of locations along the path of travel in order to detect when the deskewing is complete.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for correcting the skew of a segment advancing towards a processing station.
Another object is to provide a method and apparatus for deskewing that relies on only two aligned sensing points to resolve an out-of-alignment situation.
A further object is to provide a deskewing method and apparatus that allows a segment to pivot about a fixed pivot point without damaging, wrinkling, or bowing the segment.
Yet another object is to provide a deskewing method and apparatus that accomplishes a deskewing operation by repeatedly reacquiring a single fiducial mark during the course of a deskewing operation.